Harry at Crooked Timber has a fascinating post on the differences between high-performing schools in affluent districts, and those in high-poverty districts.  The schools in affluent districts view differences between student performance as a sign of differing ability, and rely on parents and students to fix problems through outside work and services.  The schools in poor districts, on the other hand, are intensively focused on bringing up the work of the bottom kids through team efforts and systematic analyses of how the teaching is working.

Harry identifies two reasons this should worry us:

I think it is more worthy of attention than Laura says, for two reasons. First, these schools are typically lavished with public money, relative to other schools which could make much better use of that money. States should be shifting money from such schools to schools with high-need students, and using at least part of that money to fund reform and improvement efforts. Second, these schools typically have some, and sometimes have a good number, of students from low-income families; and these students are typically seen just as problems, and are in the lousy situation of being in a school where their achievement doesn't matter much. KTM points to this excellent paper by Paul Attewell arguing that in affluent "star" schools attention is lavished on those most likely to attend Ivy League colleges, at the expense of all lower-achieving students. (Attewell's paper was written prior to implementation of NCLB, and it would be interesting to see whether the dynamics he identifies have changed at all).

I'd add a third reason:  those schools are often the model for schools in poor districts.  The affluent assume that what works in their school district, for their children, must be what works, and vote, and donate, accordingly.